Hydrosilylation reaction involving addition reaction of a Si—H functionality compound to a compound having a carbon-carbon double or triple bond is a useful means for synthesizing organosilicon compounds and is also industrially important synthetic reaction.
Pt, Pd and Rh compounds are known as catalysts for the hydrosilylation reaction. Most often used among them are Pt compounds as typified by Speier catalysts and Karstedt catalysts.
One of problems associated with Pt compound-catalyzed reactions is that the addition of a Si—H functionality compound to terminal olefin entails side reaction or internal rearrangement of olefin. Since this system does not display addition reactivity to internal olefin, unreacted olefin is left in the addition product. To complete the reaction, the olefin must be used previously in excess by taking into account the portion that is left behind due to side reaction.
Another problem is low selectivity between α- and β-adducts depending on the identity of olefin.
The most serious problem is that all Pt, Pd and Rh as the center metal are very expensive noble metal elements. Since metal compound catalysts which can be used at lower cost are desired, a number of research works have been made thereon.
For example, reaction in the presence of iron-carbonyl complexes such as Fe(CO)5 and Fe3(CO)12 is known from Non-Patent Document 1. For this reaction, reaction conditions including a high temperature of 160° C., or light irradiation is necessary (Non-Patent Document 2).
Non-Patent Document 3 reports exemplary reaction of methylvinyldisiloxane with methylhydrogendisiloxane using an iron-carbonyl complex having a cyclopentadienyl group as ligand. In this reaction, dehydrogenation silylation reaction takes place preferentially.
Non-Patent Document 4 describes reaction using an iron catalyst having a pyridine ligand. A large excess of reducing agent (NaBHEt3) is necessary as reaction aid. Although PhSiH3 and Ph2SiH2 add to olefins, more useful trialkylsilanes, alkoxysilanes and siloxanes have poor addition reactivity to olefins.
Non-Patent Documents 5 and 6 report Fe complexes having a bisiminopyridine ligand. It is disclosed that they display good reactivity to alkoxysilanes and siloxanes under mild conditions. The reaction using these complexes, however, has several problems including low reactivity to internal olefin, use of sodium amalgam, which consists of water-prohibitive sodium and highly toxic mercury and requires careful handling, or use of water-prohibitive NaBEt3H during the synthesis of the complex, and low stability of the complex compound itself, which requires handling in a special equipment like glovebox and storage in nitrogen atmosphere.
On the other hand, a number of reports are made on hydrogenation reaction of olefins. For example, Non-Patent Document 7 reports hydrogenation by thermal reaction using Fe(CO)5 catalyst, and Non-Patent Document 8 reports hydrogenation by photo-reaction. However, the thermal reaction requires high-temperature (180° C.) and high-pressure (28 atm.) conditions, and the turnover number is as low as 0.5. It is not concluded that the catalyst has sufficient activity. Also the photo-reaction can take place even at room temperature, but a turnover number of 33 is still insufficient.
Non-Patent Document 9 reports exemplary iron-catalyzed reaction using an organoaluminum compound as a cocatalyst. A turnover number of 17 indicates low catalytic activity.
Non-Patent Document 10 reports exemplary reaction using an iron chloride catalyst in combination with a Grignard reagent as a cocatalyst. The system allows reaction to run at room temperature, but requires high-pressure (20 atm.) conditions, and the turnover number is as low as 20.
Non-Patent Document 11 reports an iron catalyst having a phosphorus ligand. Although the system allows reaction to run at room temperature and a relatively low pressure (4 atm.), the reactants are limited to styrene and some alkenes, and the turnover number is not regarded sufficient.
Also, Non-Patent Document 5 cited above reports an exemplary iron catalyst having a bisiminopyridine ligand. Reactivity is satisfactory as demonstrated by a turnover number of 1,814 at room temperature and a relatively low pressure (4 atm.). This reaction suffers from problems including safety upon synthesis and stability of the relevant compound like the aforementioned iron complex having a bisiminopyridine ligand.
Also, Non-Patent Document 12 discloses an iron catalyst supported on a polymer. Despite the advantage of possible repetitive reuse, the applicable range is limited because the use of water as the solvent is essential.
Non-Patent Document 13 discloses an iron catalyst supported on a metal-organic framework (MOF). Despite the advantage of possible repetitive reuse, the catalyst precursor must be activated with strong reducing agents.
There are reported no examples where these prior art catalysts are applied to hydrogenation reaction of tri- or tetra-substituted olefins which is generally believed difficult.
Non-Patent Document 14 discloses a mononuclear iron complex which is effective for hydrogenation reaction of tetra-substituted olefins which is generally believed difficult. A long reaction time of 48 hours is necessary and the turnover number is as small as 20.
One known method for reducing carbonyl compounds is by using hydride reagents such as aluminum hydride and boron hydride or hydrogenation in the presence of noble metal catalysts. For ketones and aldehydes among carbonyl compounds, there are known hydride reagents and hydrogenation catalysts containing noble metals which allow progress of reaction under mild conditions and are stable and easy to handle. For reducing carboxylic acid derivatives such as esters and amides, the main method uses strong reducing agents such as lithium aluminum hydride and borane (Non-Patent Document 15). However, since these reducing agents are flammable, water-sensitive substances, they are awkward to handle. Also careful operation is necessary when the aluminum or boron compound is removed from the desired compound after the reaction. In addition, high pressure of hydrogen and high reaction temperature are necessary for the reduction of carboxylic acid derivatives.
There are reported many methods using methylhydrogenpolysiloxane and hydrosilane compounds which are stable in air and easy to handle, as the reducing agent. For this reaction, however, addition of strong acids or Lewis acids is necessary as well as expensive noble metal catalysts. One recent report relates to reductive reaction of carbonyl compounds in the presence of inexpensive iron catalysts. In some examples, the catalyst is applied to reductive reaction of amides that requires rigorous conditions in the prior art. While illustrative examples of the iron catalyst are given in Non-Patent Documents 16 to 21, there is a desire to have high activity catalysts displaying a greater turnover number.
Also, Non-Patent Documents 22 to 24 report mononuclear ruthenium complexes as the metal complex compound having catalytic activity to hydrosilylation reaction, hydrogenation reaction, or reductive reaction of carbonyl compounds. On use of these ruthenium complexes, any of the reactions runs under relatively mild conditions. Yet catalysts using less expensive metals are desired as mentioned above.